Brush-type DC motors have inherent design difficulties which limit the life span and reliability of the motor. Among the difficulties are brush wear, brush arcing, acoustic noise due to brush contact and rotor heat dissipation. Because of these problems with brush-type DC motors, permanent magnet brushless DC motors have been finding wide acceptance in various applications such as tape and disk drives, and aircraft and missle electromechanical actuators. Such motors have a permanent magnet rotor, typically a high coercive force material as samarium cobalt, and a stator with a multiphase coil. An inverter circuit with pairs of complementary conducting switches converts DC to AC to excite the stator coils and operate the motor.
While eliminating the problems of brush-type DC motors, the brushless DC motor presents problems of its own. The impedance of a permanent magnet brushless DC motor is resistive at low rotational speeds and becomes increasingly reactive at higher speeds. This condition leads a shift in the displacement between the rotor and stator magnetic fields with speed change, reducing motor torque. The shift, if uncorrected, impairs motor performance.
Peterson et al. U.S. Pat. No. 4,546,293 discloses such a DC motor and inverter, particularly suited for start-stop operation as in an actuator drive. The Peterson inverter controls motor torque by pulse width modulation (PWM) of the inverter. At speeds above 45% of rated speed, the phase of excitation as advanced to increase and speed range of the motor. At speeds below 45% of rated speed, the phase of excitation is not advanced. Peterson advances phase by comparing a speed related analog signal with a ramp, the slope of which is controlled by an integrator circuit.